TECHNOLOGY AREA(S): Ground/Sea Vehicles

ACQUISITION PROGRAM: PEO LCS, PMS 501 LCS acquisition, and PMS 515 FF acquisition

OBJECTIVE: Research and develop chemical or non-chemical methods and processes to impart surface morphology modifications to aluminum-magnesium (Al-Mg) alloys to mitigate and increase the exfoliation corrosion resistance.

DESCRIPTION: 5000-series marine grade aluminum alloys are used in high-speed, high-performance ships and marine craft due to the many positive attributes (high strength-to-weight ratio, weld-ability, and marine corrosion resistance) of those alloys.� Initial research in aluminum alloys for marine use indicated that certain alloys and tempers could be made resistant to exfoliation corrosion.� Testing of these alloys and tempers was conducted for two years prior to acceptance of those alloys and tempers for widespread use in the U.S. Navy.� Most of the Navy platforms, however, have service lives of 20-30 years and have subsequently exhibited exfoliation corrosion.� Exfoliation is a special type of inter-granular corrosion that occurs on the elongated grain boundaries.� The corrosion product that forms has a greater volume than the volume of the parent metal.� The increased volume forces the layers apart, and causes the metal to exfoliate or delaminate.� Innovative approaches for processes that protect against exfoliation are needed.� Currently, when exfoliation corrosion occurs, the Navy must remove and replace the affected plate, resulting in costly and time-consuming maintenance actions.� Prevention of exfoliation corrosion would produce lifecycle cost savings and increase the operational availability of ships and craft using 5000-series aluminum.

The small business must research and develop applicable technologies that can mitigate and prevent exfoliation from occurring over the expected 25-year service life of a ship or craft.� The proposed technology must improve the exfoliation resistance by 50% without reducing the strength in the material or inducing pitting or other deleterious changes to the material microstructure.� The small business will identify the gaps between potentially applicable technologies and the future requirements and discuss the research and development (R&D) and innovation needed to fill the gaps thus justifying the need for the R&D or innovation.� The proposed treatment must maintain paint adhesion and mechanical bonding durability of the Al-Mg alloy.� The goal of this effort is to prevent exfoliation corrosion from occurring over the expected 25-year service life of a ship or craft.

Technologies currently used to mitigate and improve the corrosion resistance of titanium, steel, stainless steel, and nickel-based alloys may be applicable.� Various technologies currently being utilized by the aerospace, nuclear, and oil industries to address issues related to exfoliation, stress corrosion cracking, fretting, and wear have shown to be effective.

PHASE I: Research and develop applicable technologies that meet the overall objective of the proposal with a focus on development, testing, and analysis of the selected technology.� The proposed research should include developing an understanding of the physical mechanisms to improve the exfoliation corrosion by altering the surface structure and morphology of the aluminum. Phase I should include technology development, required testing, technical rationale for the testing, analysis, project goals, milestones, and deliverables.� Address any hazardous material and environmental issues.� The Phase I Option, if awarded, will include the initial treatment specifications and capabilities description to prototype the proposed solution in Phase II.� Develop a Phase II plan.

PHASE II: Based on the results of Phase I and the Phase II Statement of Work (SOW), treat sample coupons and conduct short-term testing of those coupons.� Take measurements at each test point to determine if actual exfoliation or grain boundary formation occurs as predicted.� Coupon testing must occur at an ISO 9001:2015 certified facility.

The developed technology must be demonstrated on 5XXX aluminum.� Samples must be treated, tested, and evaluated for exfoliation corrosion.� Corrosion testing must be conducted at an ISO 9001:2015 certified facility.� Evaluation of test samples must include on-site monitoring to determine if any corrosion occurs during testing.� Test samples must also be inspected and evaluated to determine if the treatment adversely affects material and metallurgical properties.� Test samples must be evaluated to assess the paint adhesion and mechanical bonding durability of the technology.� The Phase II test results must be used to optimize the technology for a production environment.� Develop a Phase III plan for technology transition to the Navy.

PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the technology to Navy use.� Identify all hardware, and develop all use documentation necessary to implement the technology at manufacturing facilities.� The proposed technology is applicable to ship classes including the Littoral Combat Ship (LCS), Ship to Shore Connector (SSC), and Ticonderoga-class (CG-47).

Civilian applications in the marine and oil servicing industries are possible.

The proposal surface technology has the potential for commercial applications in the aerospace, nuclear, and oil industries.� Various surface treatment technologies have been developed and used to address issues with exfoliation, stress corrosion crack, fretting, and wear in titanium, steel stainless steel, and nickel-based alloys.

REFERENCES:

1. Brosi, J.K., et al. �Delamination of Sensitized Al-Mg Alloy During Fatigue Crack Growth in Room Temperature Air.� Metallurgical and Materials Transactions A, Vol. 34A, November 2012, 3952-3956. https://www.researchgate.net/publication/235355015_Delamination_of_Sensitized_Al-Mg_Alloy_During_Fatigue_Crack_Growth_in_Room_Temperature_Air

2. Mohsen, S. et al. �Grain Orientation Effects on Delamination During Fatigue of a Sensitized Al-Mg Alloy.� Philosophical Magazine Letters, Vol 95, Issue 11, Nov 2015, 526-533. http://www.tandfonline.com/doi/abs/10.1080/14786435.2015.1110630?journalCode=tphl20

3. Liao, M. et al. �Effects of Ultrasonic Impact Treatment on Fatigue Behavior of Naturally Exfoliated Aluminum Alloys.� International Journal of Fatigue, 30 (2008), 717-726. http://www.sciencedirect.com/science/article/pii/S0142112307001715

KEYWORDS: Exfoliation Corrosion of Aluminum; Corrosion Resistant Surface Treatments; Preventing Exfoliation Corrosion; Corrosion of Marine Grade Aluminum Alloys; Corrosion Failure Mechanisms for 5000-series Aluminum; Service Life Limiting Factors for 5000-series Aluminum